This invention relates to the field of full travel keyboards. More particularly, this invention relates to key configurations for full travel keyboards of the high speed data entry type.
High speed data entry type keyboards are typically used in conjunction with computers or word processors. The requirements for keyboards of this type are such that not only must the keyboard close a unique circuit or generate a unique electrical signal for each key, but the keyboard must do so in a way that is comfortable for the keyboard operator to use. An operator may enter data via a keyboard of this type continuously for long periods of time, often for stretches of several hours at a time. It is, therefore, extremely important and a significant part of the value of a keyboard of this type that it be operable comfortably and reliably for a sustained period of time. The variables involved in the issues of comfort and reliability of such keyboards in use are generally referred to in the art as the "human factors" variables. These human factor variables typically consist of: (a) the total amount of key travel (i.e., from the beginning of the stroke to the end of the stroke of the key); (b) the amount of pretravel (i.e., the travel of the key prior to closure of the switch); (c) the location of the actuation or "fire" point (i.e., the point at which the switch is actuated to generate the electrical signal); (d) the amount of overtravel of the key (i.e., the travel of the key after the switch has been closed; and (e) discontinuity of the force versus displacement curve in the operation of the keyboard (i.e., whether or not the actuation cycle provides tactile feedback to the operator). Springs of various types and arranged in various cooperating arrays are conventionally used to provide the preferred set of human factors values.
One of the more commonly known prior art switch types of the full travel category is the capacitance type. In capacitance type switches, a thin aluminum foil disk connected to the key stem is mechanically introduced into an electrical field between two electrical conductors, which serve as the plates of a capacitor. The introduction of the conductive disk into the electric field between the conductors instantaneously reduces the dielectric constant of the space between the conductors. The conductor pairs are arranged in a matrix, the location of each of which is unique. Thus, by the technique of change in dielectric constant upon actuation of key, a signal unique to each key may be generated, sensed and electronically converted to a standard digital code. Keys of this type are reliable and have long life. However, they are relatively expensive and are characterized by having a large number of mechanical parts.
Membrane switches are another technology whose use has been considered or even found application in full travel high speed data entry keyboards. A membrane switch is a very thin, essentially planar, element. Typically, a membrane switch consists of sheets of insulating plastic with conductor patterns formed thereon by one or more conventional printed circuit techniques. In a high speed data entry type keyboard, individual mechanical keys are associated with specific switch locations of the membrane elements. An important problem in the design of such keyboards is to provide the desired human factors values for pretravel, overtravel, controlled actuation point and the appropriate force versus deflection values. These considerations require special attention when it is desired to incorporate membrane switch technology (in the form of a monolithic array of unique mechanical switches arranged in a matrix) in a high speed data entry keyboard, because the use of the membrane switch configuration presents special problems with regard to these human factors values. Prior to the present invention, the approach to this problem has typically resulted in a key switch configuration which incorporates multiple springs and large numbers of plastic parts, with resultant expense. That presents a certain irony and anachronism, in that one of the original objectives of membrane switch technology was to eliminate the large number of parts and mechanical complexity and expense of key switch elements.